Pressure-While-Drilling Data Improve Reservoir Drilling Performance
- Chris Ward (Sperry-Sun) | Espen Andreassen (Statoil A/S)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1998
- Document Type
- Journal Paper
- 19 - 24
- 1998. Society of Petroleum Engineers
- 1.12.6 Drilling Data Management and Standards, 1.10 Drilling Equipment, 1.12.3 Mud logging / Surface Measurements, 1.6.3 Drilling Optimisation, 1.6.6 Directional Drilling, 1.7.5 Well Control, 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials, 1.7.1 Underbalanced Drilling, 3 Production and Well Operations, 1.7.6 Wellbore Pressure Management, 1.12.1 Measurement While Drilling, 6.3.3 Operational Safety, 1.7 Pressure Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.11.5 Drilling Hydraulics, 1.6.1 Drilling Operation Management, 1.12.5 Real Time Data Transmission, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 5.1.2 Faults and Fracture Characterisation
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A new downhole annular pressure-while-drilling (PWD) tool has been applied recently to assist in drilling the reservoir section on the Statfjord field, offshore Norway. For drilling success in these high-angle wells it is critical to maintain the mud weight and equivalent circulating density (ECD) within safe operating limits defined by the formation fluid, collapse, and fracture pressures. Operating outside these limits historically has led to expensive lost circulation, differential sticking, and packoff incidents. Monitoring the actual downhole pressure in real time with a PWD tool, rather than relying on inferred pressures from predictive models, has allowed the operator to stay within and to better define these operating limits. The operator used this improved hydraulics information to avoid pressure-related hole problems, to optimize drilling practices, to test hydraulics models, and to obtain a greater understanding of the formation pressure limits.
The majority of drilling downtime results from hole problems principally including lost circulation, formation fluid influx, hole collapse, differential sticking, and poor hole cleaning. Such events usually lead to time-consuming and expensive incidents such as lost mud and downhole tools, well-control incidents, stuck pipe, and stuck casing. It has been estimated that these problems account for about 10 to 15% of drilling operations time in the North Sea.
Most drilling hole problems occur when the safe operating pressure limits are exceeded. These limits are defined by the pore, collapse, and fracture pressures. They are typically determined from offset well data, either by modeling or by certain measurements made while drilling the well (e.g., leakoff tests, formation tests). If the stresses imposed during drilling are allowed to exceed safe pressure limits, hole problems are likely to occur. The pressure imposed is defined by the mud weight plus or minus any dynamic pressures resulting from pipe movement (swab/surge, rotary) and fluid flow (ECD, breaking the gel strength). The static mud weight is traditionally measured at the surface, and the dynamic effects are estimated with hydraulics models.
Additional hole problems can occur if the conditions are insufficient to remove the drilled cuttings from the hole. Poor cuttings removal (hole cleaning) often results in excessive reaming times, packing off, and stuck pipe.
This paper describes the development and use of a PWD tool that directly measures the actual stresses imposed on the formation while drilling, and can give indications of the suspended cuttings load. The results, collected over a period of nearly 2 years, have produced a large amount of data. Some of the different hydraulics phenomena observed are discussed, plus an example of how the PWD tool has been used to change drilling practices and improve drilling performance on the Statfjord field. Finally, results are compared with standard hydraulics models (Bingham, yield power law) and an operator in-house program (MudCalc).
The Statfjord Field
More than 140 wells have been drilled from the three Statfjord platforms. These are producing from three reservoirs: the Brent group, the Statfjord formation, and, more recently, the Dunlin group. Many of these wells are extended-reach-drilling (ERD) wells, including several previous world records.1-3 The large number of high-inclination, ERD, and horizontal wells have required a strong focus on hole cleaning and ECD considerations. In particular, the Brent group is a pressure-depleted, Jurassic sandstone reservoir that has occasional brittle coal layers. These coal layers require special care not to exceed their low fracture strength. At the same time, the mud weight must exceed the collapse pressure of interbedded shales (Fig. 1). On several occasions, the coal has fractured and severe mud losses that were difficult to cure occurred. Recent field drilling has concentrated on the eastern crest where a faulted disorganized zone has more uncertain geology and pressures exacerbating this problem.
An operator in-house hydraulic simulation program (MudCalc) has historically proved useful in avoiding extreme ECD values arising from viscous-mud rheologies. It has helped establish ECD limits for these coal layers based on historical fracturing incidents. The program takes into account a simple hole/string geometry, bit-pressure loss, assumed pressure loss through BHA components, mud weight, and rheological properties. Missing from this model have been temperature and pressure effects on mud properties, eccentric pipe position, breaking of mud gels, overgauge hole, lateral pipe movement, and pipe rotation. New drilling fluids based on synthetic oils may also have varying density and rheological properties under downhole conditions that have proved difficult to model in realistic drilling situations. As a result of these known shortcomings of the available hydraulics models and the desire to verify the calculated ECD, it was decided to run a recorded PWD tool.
PWD Tool History
The PWD tool used for this work was originally developed in Canada for underbalanced drilling applications. Annular fluid is ported through a drill collar to a downhole recording pressure gauge that, in later versions, was connected to a measurement-while-drilling (MWD) tool for real-time data transmission. Ruggedized versions of gauges originally developed for production services are used. These are temperature compensated and have an accuracy of 10 psi over a 0-to-20,000-psi pressure range. The tools and gauges have proved to be very reliable, and calibrations have shown that data quality has always been well within specifications. Recorded and real-time versions have now been developed for 3 1/4-, 4 3/4-, 6 3/4-, 8-, and 9 1/2-in. collar sizes. Raw absolute pressures are converted to an equivalent mud weight (EMW) by use of survey and depth information, and the sensor is typically placed 5 to 30 m behind the bit. Higher pressure losses would be expected at the bit, but it has been calculated that the pressure loss between bit and sensor is negligible.
The capability to measure downhole pressures has been in many MWD tools for some time, but use has largely been restricted to measuring the differential pressure across the BHA for monitoring motor and MWD performance.
Recorded downhole pressure gauges have occasionally been run to verify hydraulics models in particular situations.4-10 However, over the past 2 years, as increasing PWD data have been collected, the value of this information has been demonstrated and real-time PWD tools have become commercial.
It was apparent from the initial PWD measurements that existing assumptions and hydraulics models did not fully account for all the phenomena observed. This often led to an underestimation of the pressures imposed on the formation. The following examples are from the Statfjord field, with a few additional examples from the nearby Gullfaks field (Figs. 2 through 10). Tables 1 and 2 show details of the well geometries and mud properties.
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